The present invention relates to apparatus which is synchronized with a frequency generator and periodically derives an updated ratio between time-varying signals being generated by an analytical instrument. Signal ratiometers are commonly known in the art of instrumentation, but few of these monitor the ratio continuously for applications where signal levels vary with time. In those continuously monitoring ratiometers which are known to the art, the ratio determined is between one time-varying signal and one fixed level signal, so that these ratiometers are useless in applications where both the numerator and denominator of the ratio relate to time-varying signals. Furthermore, error in the ratios derived with most known continuously monitoring ratiometers is caused by noise pickup from the power line energizing the instrument which generates the time-varying signal.